Balancing network

ABSTRACT

A balancing network has three planar lines which are coupled together. One end of the first line which runs between the second and the third line acts as an unbalanced gate. The opposite end of the first line is connected to earth, and one end of each of the other two lines forms a balanced gate. The balancing network is decoupled from the direct current source. The ends of the second and third lines, which do not act as gates, are capacitively coupled with one another.

BACKGROUND OF THE INVENTION

The present invention concerns a balun, consisting of planar linescoupled to each other, in which one end of a first line running betweena second and a third line serves as unbalanced signal input and theother end of this first line is contacted with ground and one end eachof the other two lines forms a balanced signal input.

Baluns are known to represent transitions between balanced andunbalanced transmission lines. A balanced line exists when the signaltransmitted over it does not have ground as reference potential. On theother hand, an unbalanced transmission line is in contact with ground onone side, so that a signal transmitted over it has ground as referencepotential. Baluns of this type are used, for example, at the inputs andoutputs of quadruplex mixers or amplifiers or modulators, etc. A balun,consisting of three coupled planar lines, is known, for example, from J.Villemazet, J. Dubouloy, M. Soulard, J. Cayrou, E. Husse, B. Cogo, J.Cazaus: New Compact Double Balanced Monolithic Down-ConverterApplication to a Single Chip MMIC Receiver for Satellite Equipment, IEEEMTT-S Digest, 1998, pages 853-856. An unbalanced gate is situated on oneend of the middle of the three lines. The other end of the middle lineis connected to ground.

The end of one of the two outer lines lying next to this line end incontact with ground is also in contact with ground, and its other endforms a balanced gate. One end of the other outer line is also incontact with ground, and the other end forms a second balanced gate. Inthis known balun, three line ends must be contacted with ground, forwhich purpose several contacts must be provided on a substrate carryingthe line, which require relatively much space on the substrate. In orderfor the inputs of the balun not to be DC short-circuited relative toground, capacitances must be inserted at all inputs for DC decoupling.

The underlying task of the invention is to provide a balun of the typejust mentioned, in which DC decoupling is accomplished with the simplestpossible means.

SUMMARY OF THE INVENTION

The mentioned task is solved with the features of Claim 1, in that, ofthe three planar lines connected to each other, one end of a first linerunning between a second and a third line serves as unbalanced gate andthe other end of this first line is contacted with ground, and one endeach of the other lines forms a balanced gate. DC decoupling is achievedby the fact that the ends of the secondhand third lines not serving asgates are capacitively coupled to each other.

As can be deduced from the subclaims, capacitive coupling of the lineend can be accomplished very simply by the fact that the ends of thesecond and third lines are connected to line sections that run next toeach other over a stipulated length, or by the fact that the ends of thesecond and third lines are connected to each other via one or morecapacitors. It is expedient to connect a capacitor in series with thethird line. This capacitor improves the balance between the balancedgates. It serves to balance the phase difference at 180°.

DRAWING

The invention is now further explained with reference to two practicalexamples depicted in the drawing. In the drawing:

FIG. 1 shows a balun, in which two lines are capacitively coupled toeach other by parallel guiding, and

FIG. 2 shows a balun, in which two lines are coupled to each other viaconcentrated capacitors.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The balun depicted in FIG. 1 consists of three planar lines 1, 2 and 3running next to each other and coupled to each other. The couplinglength of these three lines corresponds to about one-fourth of theaverage operating wavelength of the balun. The first line 1, which runsbetween the two other lines 2 and 3, is contacted with ground on oneend. For this purpose, a contact 4 to the ground surface is provided onthe bottom of the substrate on which the lines are applied. The oppositeend of this first line 1 forms an unbalanced gate 5. The end of thesecond line 2 adjacent to this unbalanced gate 5 is a first gate forbalanced signals, and the second gate 7 for balanced signals is situatedon the end of the third line 3 that is adjacent to the ground connectionof the first line 1.

The connection between the ground-contacted end of the first line 1 tocontact 4 occurs via an air gap 8 that spans the end of the third line 3without contact. The ends of the second line 2 and the third line 3opposite gates 6 and 7 are each connected to a line section 9, 10. Theline sections 9 and 10 that are adjacent to each other at opposite endsof the two lines 2 and 3 are returned in space-saving fashion to thecenter of the balun and run next to each other over a stipulated length,in which they cross the three lines 1, 2, 3 via air gaps 11 and 12without contact. The coupling length of the two line sections 9 and 10is chosen so that a desired capacitive coupling is produced between theends of the two lines 2 and 3. This capacitive coupling of the two lines2 and 3 means that the balun is DC-decoupled.

As shown in FIG. 1, a capacitor C1 is connected in series to the thirdline 3 in the region of gate 7. This capacitor C1 improves the balancebetween the balanced gates 6 and 7. It serves to balance the phasedifference between the two gates 6 and 7 at 180°.

A balun is depicted in FIG. 2 that has essentially the same design asthe balun of FIG. 1 and therefore also the same reference numbers. Thepractical examples of a balun depicted in FIGS. 1 and 2 differ in thetype of capacitive coupling between the ends of the two lines 2 and 3.Whereas, according to the practical example in FIG. 1, capacitivecoupling occurs via a line coupling between line sections 9 and 10, inthe balun according to FIG. 2 the two line sections 9 and 10 areconnected to each other via two capacitors C2 and C3, designed asconcentrated components. The two capacitors C2 and C3, connected to theends of the line sections 9 and 10, which are arranged on both sides ofthe lines 1, 2 and 3 coupled to each other, are connected to each othervia an air gap 13 that crosses the three lines 1, 2, and 3 withoutcontact. Instead of two capacitors, only one capacitor can also beinserted between the line sections 9 and 10.

What is claimed is:
 1. A balun, comprising: a plurality of planar linescoupled to each other, one end of a first of the lines interposedbetween a second of the lines and a third of the lines serving as anunbalanced gate, the other end of the first line being connected toground, one end of each of the second and third lines serving as abalanced gate, and the other ends of the second and third lines, notserving as gates, being capacitively coupled to each other.
 2. The balunaccording to claim 1, wherein, for capacitive coupling, the other endsof the second and third lines are connected to line sections that runnext to each other over a stipulated length.
 3. The balun according toclaim 1, wherein, for capacitive coupling, the other ends of the secondand third lines are connected to each other via at least one capacitor.4. The balun according to claim 1, and a capacitor connected in seriesto the third line.
 5. The balun according to claim 1, wherein the first,second and third lines coupled to each other have a coupling length ofabout one-fourth of an average operating wavelength of the balun.